The mosque that Orlando nightclub gunman Omar Mateen attended was set on fire in what Muslim leaders say was the latest incident in an escalating campaign of harassment and violence against the house of worship and its members.

The mosque that Orlando nightclub gunman Omar Mateen attended was set on fire in what Muslim leaders say was the latest incident in an escalating campaign of harassment and violence against the house of worship and its members.

Given the timing - Sunday's 15th anniversary of 9/11 and the start of the Muslim holiday Eid al-Adha - investigators believe the blaze at the Islamic Centre of Fort Pierce, Florida, may have been a hate crime, sheriff's spokesman Major David Thompson said.

No one was injured. The fire burned a 10-by-10-foot hole in the roof at the back of the mosque's main building and blackened its eaves with soot.

A surveillance video from the mosque showed a man approaching the building at around 12.30am local time with a bottle of liquid and some papers, then leaving when there was a flash and shaking his hand as though he may have burned it, Thompson said.

Mateen was killed by police after opening fire at the Pulse nightclub on June 12 in a rampage that left 49 victims dead and 53 wounded. His father is among roughly 100 people who attend the mosque.

Wilfredo Amr Ruiz, Florida's director of the Council on American-Islamic Relations (CAIR), said the mosque and its worshippers have been harassed since the massacre.

"First there were threatening voicemails," he said. "Then drivers would splash water on the parishioners leaving on Fridays, and then a member got beat up in the parking lot when he came to the mosque for early morning prayers, and now the mosque has been set on fire."

The FBI and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives joined the investigation into the fire. Officials said they will release the video and ask for the public's help in identifying the arsonist.

Ariana Borras stopped by the mosque with her two-year-old son to pay respects on Monday morning. Ms Borras said she is Hispanic and not a Muslim but is sometimes asked if she is Muslim because of the colour of her skin.

The fire "makes me scared for my son. It makes me scared for my skin colour," she said. "There have been a lot of racial issues going on and there's so much hate in the world right now."

Michael Parsons, 22, was standing outside his parents' home, directly across the street from the mosque. He said that since the Orlando shooting, "a lot of people have been driving by hollering and yelling expletives at the church or mosque or whatever they call it."

Parsons, a gutter installer with "trust no one" tattooed on his chest, said his mother's cancer doctor attends the mosque.

"America was founded so people can believe what they want to believe and do what they want to do," he said. "These guys flying the American flag on their trucks don't really know what the freedom is they're fighting for."

On July 2, a few weeks after the nightclub massacre, a man was beaten outside the mosque. CAIR said the victim was a Muslim and the attacker hurled slurs. A suspect was arrested. CAIR said that earlier that day, a man in a truck stopped outside the mosque and said: "You Muslims need to get back to your country."

On Monday morning, a truck with a "Don't Tread on Me" bumper sticker drove past the mosque. The driver revved his engine and raised his index finger in the air as if making the gesture for the number one.